Plumb bobs are ancient tools used in the building trades and other activities to locate a point in space directly below or directly above another point in space. Conventional plumb bobs are typically brass, steel or combinations of metals, with a pointer or tip on one end and an arrangement for attachment of a string or cord to the other end. The plumb bob is suspended from the cord, and after all motion stops, the tip of the pointer is located directly below the point in space to which the remote end of the cord is attached and from which the plumb bob is suspended.
Conventional plumb bobs are typically symmetrical about a longitudinal axis on which the plumb bob tip and cord attachment point are both located, often having a round, hexagonal or octagonal shape. Conventional plumb bobs generally have a bulbous portion that is often relatively close to the cord attachment point. Such bulbous portions are included in order to ensure that the plumb bob has relatively substantial mass despite a modest length. Modest length is desirable so that unwanted pendular, swinging, or wobbling motion of the plumb bob from the cord-to-plumb bob attachment point dies out quickly. Substantial mass is desirable for accuracy.
Plumb bobs are often used in locations well separated from other structures. However, among the most frequent uses of plumb bobs are ones immediately adjacent to a wall or other vertical structure, where the plumb bob is used to locate a point on a floor or baseboard immediately below another point higher on or at the top of the wall. In other instances they are used when such a higher point needs to be located immediately above an established point on the floor or baseboard.
Both round and polygon cross-section plumb bobs roll around against the wall or baseboard in such applications, making it very difficult to establish an accurate position and sorely trying the user's patience. Even when such round or polygon plumb bobs stabilize, the plumb bob tip is no closer than half the bob's diameter away from the adjacent wall, making it very difficult to project the position of the tip onto the wall accurately. Furthermore, the bulbous body of most plumb bobs makes it impossible even to see the plumb bob tip from the user's typical position generally above the suspended bob.
Two other recurring problems are associated with conventional plumb bobs. First, their cords frequently become tangled when the plumb bob is not in use, particularly if stored in a typical tool box, and the cord frequently acquires permanent knots and loops intended to be only temporary for hanging the bob during use from a nail or other object. Second, sharp plumb bob tips are desirable for accuracy but are frequently damaged during intervals of tool box storage and occasionally impale the hand of a tool box visitor.